The present invention relates to a process for the preparation of a mixture of low-molecular weight polyhydric alcohols by the catalytic hydrogenation of formose. Formose is a mixture of low-molecular weight hydroxyaldehydes, hydroxyketones and polyhydric alcohols which is formed in the condensation of formaldehyde.
Since the work of Butlerow and Loew (Ann. 120, 295 (1861) and J. prakt. Chem. 33, 321 (1886)) hydroxyaldehydes and hydroxyketones have been formed by condensation of formaldehyde hydrate (formose synthesis). Formose has also been prepared by condensation of formaldehyde in the presence of lead-II compounds and of compounds capable of enediol formation, at a temperature of 70.degree.-110.degree. C. In this type of condensation, control of the pH of the reaction mixture is considered to be essential. Such pH control was generally achieved by the addition of an inorganic or organic base (see, e.g., German Offenlegungsschrift No. 2,639,084). In addition to formaldehyde, formose may also be prepared from mixtures of low-molecular hydroxyaldehydes and/or hydroxyketones and, if appropriate, polyhydric alcohols.
Many attempts have been made to convert mixtures of hydroxyaldehydes, hydroxyketones and/or polyalcohols into color-stable mixtures of polyalcohols. (Such polyol mixtures obtained by reduction of formoses are hereinafter called "formitols.") Sodium amalgam was used in the earliest processes (Loew, J. prakt. Chem. 33, 325 (1886)), but sodium borohydride has been used in more recent processes (compare R. D. Partridge, A. H. Weiss and D. Todd, Carbohydrate Research, 24, 42 (1972)). The reduction may also be accomplished electrochemically.
Process for the catalytic hydrogenation of sugars have also been modified in an effort to develop a process for producing a stable polyol mixture by the reduction of formose. The reaction conditions employed in each of these processes are quite different from those of the other processes particularly with respect to the nature and amount of catalyst and the concentration of the formose employed. For example, L. Orthner and E. Gerisch (Biochem. Zeitung 259, 30 (1933)) describe a process for the catalytic hydrogenation of formose in which a 4% strength aqueous formose solution is hydrogenated with 170% by weight (relative to formose) of Raney nickel for a period of 7-8 hours at 130.degree. C. and under a hydrogen pressure of 120 bars. In some of the earlier processes which employed metal catalysts or noble metal catalysts (especially Raney nickel), hydrogenation in an alkaline medium was strictly avoided to prevent the occurrence of side reactions and discoloration (W. Langenbeck, J.f. prakt. Chemie 3, 206 (1956)).
None of these processes have, however, proven to be economically practicable. One possible explanation for this failure to develop an economical process for reducing formose by adaptation of known sugar processes is that due to the exceptional polymolecularity of formose, formose is substantially different from sugars.
Another reason that known formose-producing processes are uneconomical lies in the fact that basic inorganic salts such as Ca salts and Pb salts, are used as catalysts. These salts must be removed before the hydrogenation, especially the lead salts because lead is a powerful catalyst poison (see M. Freifelder, Practical Catalytic Hydrogenation, pages 24, 25, Wiley, New York (1971) and the literature quoted therein). Removal of these salts complicates the process and increases the cost. Methods for removing lead which have been developed include ion exchange (U.S. Pat. No. 2,775,621) and precipitation (U.S. Pat. Nos. 2,276,192 and 2,271,083) before the hydrogenation. In German Patent Specification No. 830,951 (page 1, lines 7-23), the troublesome calcium and lead ions are precipitated as carbonates to prevent undesired side reactions in the hydrogenation and poisoning of the Raney nickel catalyst. Complete deionization is not achieved in the latter procedure (though presumably almost all of the lead is removed) so the residual salt content is removed by ion exchange after the hydrogenation.
It would therefore be advantageous to develop a process for producing a mixture of polyhydroxyl compounds from formose which does not require the removal of the calcium and/or lead salt catalysts used in producing the formose.